In the field of steel sheets for automobile use, to achieve both lighter weight for improved fuel efficiency and improvement of the impact resistance, there has been growing use of high strength steel sheet having a high tensile strength. However, along with higher strength, the press formability of steel sheet falls, so production of complicated shapes of products has become difficult.
As a result, for example, along with the higher strength of steel sheet, the problem of the ductility falling and fracture occurring at portions with a high working degree and the problem of the springback and wall camber becoming greater and therefore the dimensional precision deteriorating arise. Therefore, it has not been easy to press-form steel sheet having a high strength, in particular 780 MPa or more tensile strength, into a product having a complicated shape.
Therefore, in recent years, as disclosed in PLT 1, as art for press-forming high strength steel sheet and other such hard-to-shape materials, hot stamping has been employed. “Hot stamping” is a hot forming technique which heats a material used for forming and then forms it. With this technique, the sheet is hardened simultaneously with the forming process, so at the time of the forming process, the steel sheet is soft and has good shapeability while after the forming process, the shaped member can be given a strength higher than steel sheet for cold forming use.    PLT 2 discloses a steel member having a 980 MPa tensile strength.    PLT 3 discloses to lower the cleanliness and segregation ratios of P and S to obtain a hot pressed steel sheet member excellent in strength and toughness.